Method of manufacturing a dual wafer tunneling gyroscope

ABSTRACT

A method of making a micro electromechanical gyroscope. A cantilevered beam structure, first portions of side drive electrodes and a mating structure are defined on a first substrate or wafer; and at least one contact structure, second portions of the side drive electrodes and a mating structure are defined on a second substrate or wafer, the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer and the first and second portions of the side drive electrodes being of a complementary shape to each other. A bonding layer, preferably a eutectic bonding layer, is provided on at least one of the mating structures and one or the first and second portions of the side drive electrodes. The mating structure of the first substrate is moved into a confronting relationship with the mating structure of the second substrate or wafer. Pressure is applied between the two substrates so as to cause a bond to occur between the two mating structures at the bonding or eutectic layer and also between the first and second portions of the side drive electrodes to cause a bond to occur therebetween. Then the first substrate or wafer is removed to free the cantilevered beam structure for movement relative to the second substrate or wafer. The bonds are preferably eutectic bonds.

TECHNICAL FIELD The present invention relates to micro electromechanical(MEM) gyroscopes using dual wafers which are bonded together preferablyeutectically.

[0001] 1. Related Applications

[0002] This invention is related to other inventions which the subjectof separate patent applications filed thereon. See: U.S. patentapplication Ser. No. ______ entitled “A Single Crystal, Dual Wafer,Tunneling Sensor or Switch with Silicon on Insulator Substrate and aMethod of Making Same” (attorney docket 617965-3); U.S. patentapplication Ser. No. ______ entitled “A Single Crystal, Dual Wafer,Tunneling Sensor and a Method of Making Same” (attorney docket617975-0); U.S. patent application Ser. No. entitled “A Single Crystal,Dual Wafer, Tunneling Sensor or Switch with Substrate Protrusion and aMethod of Making Same” (attorney docket 617337-2); and U.S. patentapplication Ser. No. ______ entitled “A Single Crystal, Tunneling andCapacitive, Three-Axes Sensor Using Eutectic Bonding and a Method ofMaking Same” (attorney docket 617808-9), all of which applications havethe same filing date as this application, and all of which applicationsare hereby incorporated herein by reference.

[0003] 2. Background of the Invention

[0004] The present invention provides a new process of fabricating asingle crystal silicon MEM gyroscopes using low-cost bulk micromachiningtechniques while providing the advantages of surface micromachining. Theprior art, in terms of surface micromachining, uses e-beam evaporatedmetal that is patterned on a silicon dioxide (SiO₂) layer to form thecontrol, self-test, and tip electrodes of a tunneling MEM sensor. Acantilevered beam is then formed over the electrodes using a sacrificialresist layer, a plating seed layer, a resist mold, and metalelectroplating. Finally, the sacrificial layer is removed using a seriesof chemical etchants. The prior art for bulk micromachining has utilizedeither mechanical pins and/or epoxy for the assembly of multi-Si waferstacks, a multi-Si wafer stack using metal-to-metal bonding and anactive sandwiched membrane of silicon nitride and metal, or a dissolvedwafer process on quartz substrates (Si-on-quartz) using anodic bonding.None of these bulk micromachining processes allow one to fabricate asingle crystal Si cantilever (with no deposited layers over broad areason the beam which can produce thermally mismatched expansioncoefficients) above a set of tunneling electrodes on a Si substrate andalso electrically connect the cantilever to pads located on thesubstrate. The fabrication techniques described herein provide thesecapabilities in addition to providing a low temperature process so thatCMOS circuitry can be fabricated in the Si substrate before the MEMSsensors are added. Finally, the use of single crystal Si for thecantilever provides for improved process reproductibility forcontrolling the stress and device geometry.

[0005] MEM gyroscopes may be used in various military, navigation,automotive, and space applications. Space applications include satellitestabilization in which MEM technology can significantly reduce the cost,power, and weight of the presently used gyroscopic systems. Automotiveair bag deployment, ride control, and anti-lock brake systems provideother applications for MEM gyroscopes and/or sensor. Militaryapplications include low drift gyros.

BRIEF DESCRIPTION OF THE INVENTION

[0006] Generally speaking, the present invention provides a method ofmaking a micro electro-mechanical (MEM) gyroscope wherein a cantileveredbeam structure and a mating structure are defined on a first substrateor wafer and at least one contact structure and a mating structure aredefined on a second substrate or wafer. The mating structure on thesecond substrate or wafer is of a complementary shape to the matingstructures on the first substrate or wafer. A bonding or eutectic layeris provided on at least one of the mating structures and the matingstructure are moved into a confronting relationship with each other.Pressure is then applied between the two substrates and heat may also beapplied so as to cause a bond to occur between the two mating structuresat the bonding or eutectic layer. Then the first substrate or wafer isremoved to free the cantilevered beam structure for movement relative tothe second substrate or wafer. The bonding or eutectic layer alsoprovides a convenient electrical path to the cantilevered beam formaking a circuit with the contact formed on the cantilevered beam.

[0007] In another aspect, the present invention provides an assembly orassemblies for making a single crystal silicon MEM sensor therefrom. Afirst substrate or wafer is provided upon which is defined a beamstructure and a mating structure. A second substrate or wafer isprovided upon which is defined at least one contact structure and amating structure, the mating structure on the second substrate or waferbeing of a complementary shape to the mating structure on the firstsubstrate or wafer. A pressure and heat sensitive bonding layer isdisposed on at least one of the mating structures for bonding the matingstructure defined on the first substrate or wafer with the matingstructure on the second substrate in response to the application ofpressure and heat therebetween.

[0008] In operation, a Coriolis force is produced normal to the plane ofthe device by oscillating the beam laterally across the substrate. Theside drive electrodes are preferably fabricated with the cantileveredbeam on the first substrate and are bonded to the second substrate atthe same time that the cantilevered beam is attached. This provides forhigh alignment accuracy between the cantilevered beam and the sideelectrodes.

BRIEF DESCRIPTION OF THE FIGURES

[0009]FIGS. 1A through 6A depict the fabrication of a first embodimentof the cantilevered beam forming portion of a MEM gyroscope;

[0010]FIGS. 1B through 6B correspond to FIGS. 1A-6A, but show thecantilevered beam forming portion, during its various stages offabrication, in plan view:

[0011]FIGS. 7A through 11A show, in cross section view, the fabricationof the base portion of the gyroscope;

[0012]FIGS. 7B through 11B correspond to FIGS. 7A-9A but show thefabrication process for the base portion in plan view;

[0013]FIGS. 12 and 13 show the cantilevered beam forming portion and thebase portion being aligned with each other and being bonded togetherpreferably by eutectic bonding; and

[0014]FIGS. 14A and 15 show the completed MEM gyroscope in crosssectional view, FIG. 15 being enlarged compared to FIG. 14A;

[0015]FIG. 14B shows the completed MEM gyroscope in plan view;

[0016]FIGS. 16 and 17 show a modification of the cantilevered beamforming portion wherein the beam is formed on an etch stop layer andalso shown the base portion being aligned therewith and being bondedthereto preferably by eutectic bonding;

[0017]FIGS. 18A and 18b show the completed MEM gyroscope in crosssectional view and plan views; and

[0018]FIG. 19 shows a modification wherein a relatively small ribbonconductor is provided on the cantilevered beam.

DETAILED DESCRIPTION

[0019] Several embodiments of the invention will be described withrespect to the aforementioned figures. The first embodiment will bedescribed with reference to FIGS. 1A through 15. A second embodimentwill be discussed with reference to FIGS. 16 through 18B. Furthermodifications are described thereafter.

[0020] The MEM gyroscope shown in the accompanying figures is not drawnto scale, but rather are drawn to depict the relevant structures forthose skilled in this art. Those skilled in this art realize that thesedevices, while mechanical in nature, are very small and are typicallymanufactured using generally the same type of technology used to producesemiconductor devices. Thus a thousand or more devices might well bemanufactured at one time on a silicon wafer. To gain an appreciation ofthe small scale of these devices, the reader may wish to turn to FIG. 15which includes size information for the first embodiment of a MEMgyroscope utilizing the present invention. The figure numbers with theletter ‘A’ appended thereto are section views taken as indicated in theassociated figure numbers with the letter ‘B’ appended thereto, butgenerally speaking only those structures which occur at and immediatelyadjacent the section are shown and not structures which are well behindthe section. For example, in FIG. 2A, the portion of the mask 14 whichforms the upper arm of the letter E shaped structure seen in FIG. 2Bdoes not appear in FIG. 2A since it is located spaced from the planewhere the section is taken; however, the opening 14-3 behind the sectionline which is used to help define one of the two side drive electrodesof the gyroscope is shown. The section views are thus drawn for ease ofillustration.

[0021] Turning to FIG. 1A and 1B, a starting wafer for the fabricationof the cantilever is depicted. The starting wafer includes a wafer ofbulk n-type silicon (Si) 10 upon which is formed a thin layer of dopedp-type silicon 12. The silicon wafer 10 is preferably of a singlecrystalline structure having a <100> crystalline orientation. The p-typesilicon layer 12 is preferably grown as an epitaxial layer on siliconwafer 10. The layer 12 preferably has a thickness of in the range of 1to 20 micrometers (μm), but can have a thickness anywhere in the rangeof 0.1 μm to 800 μm. Generally speaking, the longer the cantileveredbeam is the thicker the beam is. Since layer 12 will eventually form thecantilevered beam, the thickness of layer 12 is selected to suit thelength of the beam to be formed.

[0022] Layer 12 in this embodiment is with Boron such that itsresistivity is reduced to less than 0.05 Ω-cm and is preferably doped todrop its resistivity to the range of 0.01 to 0.05 Ω-cm. The resistivityof the bulk silicon wafer or substrate 10 is preferably about 10 Ω-cm.Boron is a relatively small atom compared to silicon, and thereforeincluding it as a dopant at the levels needed (10²⁰) in order to reducethe resistivity of the layer 12 tends to induce stress which ispreferably compensated for by also doping, at a similar concentrationlevel, a non-impurity atom having a larger atom size, such as germanium.Germanium is considered a non-impurity since it neither contributes norremoves any electron carriers in the resulting material.

[0023] Layer 12 shown in FIGS. 1A and 1B is patterned using well knownphotolithographic techniques to form a mask layer 14, patterned asshown, preferably to assume the shape of a capital letter ‘E’, withmesas 14-3, which will be used to help define side drive electrodes forthe gyroscope. While the shape of the capital letter ‘E’ is preferred,other shapes can be used. In this embodiment, the outer peripheralportion of the E-shape will form a mating and supporting structure whichwill be used to join the cantilever portion of the sensor to the baseportion and to support the cantilevered beam above the base portion.

[0024] After the mask layer 14 has been patterned as shown in FIGS. 2Aand 2B, the wafer is subjected to a plasma etch in order to etch throughthe exposed thin layer of p-type doped silicon 12 and also to over etchinto the silicon wafer 10 by a distance of approximately 500 Å. Thisetching step defines the outer peripheral portion of the E-shape inlayer 12, a cantilevered beam having a thick portion 12-2 and a thinelongated portion 12-5 (see FIG. 4B) as well as portions 12-3 of theside drive electrodes.

[0025] The mask 14 shown in FIGS. 2A and 2B is then removed and anotherphotoresist layer 16 is applied, which is patterned as shown in FIGS. 3Aand 3B by providing two openings therein 16-1, 16-2 plus two openingslabeled 16-3 which align with the two small portions 12-3 of layer 12which remain due to the aforementioned etching step. Opening 16-1basically follows the outer perimeter of the ‘E’ shape of the underlyingthin layer of p-type silicon 12 while opening 16-2 is disposed at oradjacent an end of the thick portion 12-2 (FIG. 4B) of the interior legof the ‘E’-shaped p-type silicon layer 12. The interior leg 12-2, 12-5will become to the cantilevered beam.

[0026] Layers of Ti/Pt/Au are next deposited over mask 16 and throughopenings 16-1, 16-2 and 16-3 to form a post contact 18-1, a tunnellingtip contact 18-2 and two side drive electrode contacts 18-3. TheTi/Pt/Au layers preferably have a total thickness of about 2000 Å. Theindividual layers of Ti and Pt may have thicknesses in the ranges of100-200 Å and 1000-2000 Å, respectively. After removal of thephotoresist 16, the wafer is subjected to a sintering step atapproximately 520° C. to form an ohmic Ti—Si juncture between contacts18-1 and 18-2 and the underlying layer 12. As will be seen withreference to FIG. 19, the sintering step can be eliminated if a metallayer, for example, is used to connect contacts 18-1, 18-2 and 18-3.

[0027] As another alternative, which does rely on the aforementionedsintering step occurring, post contact 18-1 may be formed by layers ofTi and Au (i.e without Pt), which would involve an additional maskingstep to eliminate the Pt layer from post contact 18-1. However, in thisalternative, the sintering would cause Si to migrate into the Au to forman Au/Si eutectic at the exposed portion of post contact 18-1 shown inFIGS. 4A and 4B. As a further alternative, the exposed portion of thepost contact 18-1 shown in FIGS. 4A and 4B could simply be deposited asAu/Si eutectic, in which case the Pt layer in the post contact 18-1could be optionally included. Post contact 18-1 may be eliminated if thesubsequently described bonding between the cantilevered beam formingportion 2 and the base portion 4 occurs non-eutectically.

[0028] As a result, the exposed portion of the post contact 18-1 and theexposed portions 18-3 of the side drive electrodes 12-2, 18-3 shown inFIGS. 4A and 4B are formed preferably either by Au or by Au/Si. When thecantilevered beam forming portion 2 and the base portion 4 are mated asshown and described with reference to FIGS. 12 and 13 (and withreference to FIGS. 16 and 17 for a second embodiment) , one of theexposed mating surfaces is preferably a Au/Si eutectic while the otheris preferably Au. Thus, exposed mating surfaces 18-1, 18-3 canpreferably be either Au and Au/Si if the exposed mating surface on thebase portion 4 is the other material, i.e., preferably either Au/Si orAu so that a layer of Au/Si confronts a layer of Au.

[0029] The structures shown in FIGS. 4A and 4B are then covered with alayer of photoresist 20 which, as shown in FIG. 5A, is patterned asshown in FIGS. 5A and 5B, with a opening 20-2 therein over tunnellingtip contact 18-1. Those skilled in the art will appreciate that the sizeof the openings 16-1, 16-2, 16-3 and 20-2 are not drawn to scale on thefigures and that openings 16-2 and 20-2 would tend to be significantlysmaller than would be openings 16-1 and 16-3-1. As such, when a ratherthick layer of Au 26, preferably having a thickness of about 15,000 Å,is deposited on the wafer, it basically clogs opening 20-2 (see FIG.5A). Those skilled in the art will appreciate that there is fill-in atthe sides of a mask when a layer such as Au layer 26 is depositedbecause of an increasing overhang which occurs at the edges of opening20-2 as the deposition process proceeds. Since opening 20-2 is rathernarrow, the deposited Au 26, as shown at numeral 26-2, assumes apyramidal-like or conical-like shape as the opening is clogged with Au.The thickness of the deposition of Au layer 26 is sufficiently thick toassure that layer 26 will close across the top of opening 20-2 duringthe deposition process and so that structure 26-2 assumes its pointedconfiguration.

[0030] The photoresist 20 is then dissolved lifting off the layer 26formed thereon and leaving the structures depicted by FIGS. 6A and 6B.

[0031] The fabrication of the base portion 4 of this embodiment of theMEM gyroscope will now be described with reference to FIGS. 7A through11B. Turning to FIGS. 7A and 7B, a wafer 30 of silicon is shown uponwhich a layer of photoresist has been deposited and patterned (i) toassume preferably the outerperipheral shape of a capital letter ‘E’ 50-1complementary to the outer peripheral shape of patterned mask layer 14(FIG. 2B) and (ii) to define mesas 50-3 complementary to the size, shapeand location of the first portions 12-3, 18-3 of the side driveelectrode formed on the cantilevered beam forming portion 2. The exposedsilicon is then subjected to an etch, etching it back approximately20,000 Å, to define a protruding portion 30-1 of wafer 30 under thepatterned mask 50-1 of the photoresist and protruding portions 30-3under mesas 50-3. The photoresist mask 50 is then removed and wafer 30is oxidized to form layers of oxide 52, 54 on its exposed surfaces. Theoxide layers are each preferably about 1 μm thick. Of course, the endsurfaces shown in FIG. 8A are not shown as being oxidized because it isassumed that the pattern shown in FIG. 8A (and the other figures) isonly one of a number of repeating patterns occurring across an entirewafer 30. The oxide includes protruding portions 52-1 and 52-3 thereofon protruding portions 30-1 and 30-3 of the wafer 30.

[0032] Turning to FIGS. 9A and 9B, a layer of photoresist 56 is appliedhaving (i) an opening therein 56-1 which again assumes theouterperipheral shape of a capital letter ‘E’, as previously describedand (ii) a pair of openings 56-3 to aid in the formation of the secondportion of the side electrode on wafer 30. Then, a layer of Ti/Pt/Au 58,preferably having a thickness of 2,000 Å, is deposited through openings56-1, 56-3 followed by the deposition of a layer 60 of an Au/Si eutecticpreferably with a 1,000 Å thickness. Layers 58-1, 58-3 of Ti/Pt/Au andlayers 60-1, 60-3 of the Au/Si eutectic are thus formed. Layers 58-1 and60- preferably follow the outerperipheral shape of a capital letter ‘E’,as can be clearly seen in FIG. 9B, while layers 58-3 and 60-3 disposedon the oxided protrusion 52-3 define the second portions of the sidedrive electrodes. The second portions of the side drive electrodes willbe mated with the first portions thereof formed on cantilevered beamforming portion 2 in due course. Of course, if the post contact 18-1 andthe side electrode contacts 18-3 (see FIG. 4A) are either formed of anAu/Si eutectic or has an Au/Si eutectic disposed thereon, then layers60, 60-1, 60-3 may be formed of simply Au or simply omitted due to thepresence of Au at the exposed layers 58-1 and 58-3.

[0033] Photoresist layer 56 is then removed and a layer 62 ofphotoresist is applied and patterned to have (i) openings 62-2, 62-3,62-4 and 62-6, as shown in FIG. 10A, (ii) openings for pads 40-1 through40-5 and their associated ribbon conductors 42; (iii) an opening forguard ring 44 and its pad, as depicted in FIG. 10B. For the ease ofillustration, the opening for guard ring 44 is not shown in FIG. 10A. Alayer 38 of Ti/Pt/Au is then deposited over the patterned photoresistlayer 62 and through openings 62-2, 62-3, 62-4 and 62-6 therein formingcontacts 38-2, 38-3, 38-4 and 38-6 and the photoresist 62 is removed tothereby arrive at the structure shown in FIGS. 11A and 11B. Thosecontacts are interconnected with their associated pads 40-2 through 44-4by the aforementioned ribbon conductors 42, which contacts 40 and ribbonconductors 42 are preferably formed at the same time as contacts 38-3,38-4 and 38-2 are formed. The outerperipheral layers 58-1 and 60-1 arealso connected with pad 40-1 by an associated ribbon conductor 42. Theprotrusion 30-1, which preferably extends approximately 20,000 Å highabove the adjacent portions of wafer 30′, and the relatively thin layers58-1 and 60-1 form the mating structure for the base portion 4.

[0034] Contacts 38-6 are preferably triangularly shaped with theirhypotenuses confronting each other and positioned such that thehypotenuses will lie under a centerline of the elongated cantileveredbeam 12-5 when the cantilevered beam forming portion 2 is joined to thebase portion 4.

[0035] Pad 40-1 is connected to layers 58-1 and 60-1 and provides a padfor a beam bias voltage. Pad 40-2 is connected to tip contact 38-2 andprovides a pad for the tip contact 38-2. Pad 40-3 is connected tocontacts 38-3 and provides a pad for the side drive electrodes 38-5,58-3 and 60-3 (when the two portions 2, 4 are bonded together). Pad 40-4is connected to contact 38-4 and provides a pad for device testing. Pad40-5 is connected to contact 38-5 and provides a pad for a pull downvoltage. Pads 40-6 are connected to the two side sense contacts 38-6 andprovides pads for the side sense contacts 38-6.

[0036] Turning to FIG. 12, the cantilevered beam forming portion 2 isnow bonded to base portion 4. As is shown in FIG. 12, the two wafers 10and 30 are brought into a confronting relationship so that their matingstructures 18-1, 30-1, 58-1 and 60-1 are in alignment and so the firstand second portions of the side drive electrode are in alignment and sothat (i) layers 18-1 and 60-1 properly mate with each other and (ii)layers 18-3 and 60-3 properly mate with each other. Pressure and heat(preferably by applying a force of 5,000 N at 400° C. between three inchwafers 2, 4 having 1000 sensors disposed thereon) are applied so thateutectic bonding occurs between layers 18-1 and 60-1 and between layers18-3 and 60-3 as shown in FIG. 13. Thereafter, silicon wafer 10 isdissolved so that the MEM sensor structure shown in FIG. 14 is obtained.The p-type silicon layer 12 includes a portion 12-2 which serves as thecantilevered beam and another portion which is attached to the baseportion 4 through the underlying layers. The gold contact 26-2 iscoupled to pad 40-1 by elements 18-2, 12-2, 12-1, 18-1, 60-1, 58-1 andits associated ribbon conductor 42. If the bonding is donenon-eutectically, then higher temperatures will be required.

[0037] Protrusion 30-1 and layers 18-1, 60-1, and 58-1 have preferablyassumed the shape of the outerperpherial edge of a capital letter ‘E’and therefore the cantilevered beam of the MEM gyroscope is wellprotected by this physical shape. After performing the bonding, siliconlayer 10 is dissolved away to arrive at the resulting MEM sensor shownin FIGS. 14A and 14B. The silicon can be dissolved with ethylenediaminepyrocatechol (EDP). This leaves only the Boron doped siliconcantilevered beam 12 with its associated contact 26-2 and its supportingor mating structure 18-1 bonded to the base structure 4. Preferabledimensions for the MEM sensor are given on FIG. 15. The beam aspreferably has a length of 200 to 300 μm (0.2 to 0.3 mm).

[0038]FIG. 15 is basically identical to FIG. 14, but shows the MEMsensor in somewhat more detail and the preferred dimensions of the MEMsensor are also shown on this figure.

[0039] Instead of using EDP as the etchant, plasma etching can be usedif a thin layer 11 of SiO₂ is used, for example, as an etch stop betweenlayer 12 and substrate 10. FIGS. 16, 17, 18A and 18B are similar toFIGS. 12, 13, 14A and 14B, respectively, but differ in that a thin layerof SiO₂ is shown being utilized as an etch stop between layer 12 andsubstrate 10. Such a thin layer 11 of SiO₂ can be formed by theimplantation of oxygen so that layer 12 retains the same crystallinestructure of wafer 10. In this case the layer 12 may be undoped or maybe doped with Boron or other dopants. The plasma etch in this case is atwo step process. A first etch, which preferentially etches silicon,removes substrate 10 and a second etch, which preferentially etchesSiO₂, removes the etch stop layer 11 to arrive at the structure shown inFIGS. 18A and 18B. If layer 12 is undoped or nor sufficiently doped toprovide proper conductivity (for example, to a level less than 0.05Ω-cm), then a thin ribbon conductor 18-4 should be affixed to layer 12as shown in FIG. 19 to interconnect contacts 18-1, 18-2 and 18-3.Generally speaking, it is preferred to use the conductivity in thecantilevered beam, by sufficiently doping same, to interconnect contacts18-1, 18-2 and 18-3 rather than a separate ribbon conductor 18-4 sincethe existence of a ribbon conductor on the beam 12 may interfere withits freedom of movement in response to acceleration events which agyroscope should detect. If a ribbon conductor 18-4 is used, then isshould be kept as small as practicable in both height and width tominimize its effect on the cantilevered beam. It will be recalled thatin the embodiment of FIGS. 1A-15, that after the layer of Ti/Pt/Au 18was applied forming contacts 18-1, 18-2 and 18-3, they were sintered inorder to form an ohmic bond with Boron-doped cantilever 12. It was notedthat sintering could be avoided by providing a ribbon conductor betweenthe contacts. The just-described ribbon conductor 18-4 has the advantageof omitting any steps needed to form ohmic contacts with the beam.

[0040] It can be seen that the Si layer 12 formed on silicon wafer 10may be (i) doped with Boron or (ii) may be either undoped or doped withother impurities. If doped with Boron, layer 12 is preferably formed byepitaxial growth. If layer 12 is either undoped or doped with otherimpurities, it is preferably formed by methods other than epitaxialgrowth on substrate 10 and a thin etch stop layer 11 is then preferablyformed between the thin Si layer 12 and the silicon substrate or wafer10. This configuration is called Silicon On Insulator (SOI) and thetechniques for making an SOI structure are well known in the art andtherefor are not described in detail herein. The etch stop layer 11, ifused, is preferably a layer of SiO₂ having a thickness of about 1-2 82 mand can then be made, for example, by the implantation of oxygen intothe silicon wafer 10 through the exposed surface so as to form the etchstop layer 11 buried below the exposed surface of the silicon wafer 10and thus also define, at the same time, the thin layer of silicon 12adjacent the exposed surface. This etch stop layer 11 is used to releasethe cantilevered beam from wafer 10 by the aforementioned two stepplasma etch process. If layer 12 is doped with Boron, it is doped toreduce the resistivity of the epitaxial layer 12 to less than 1 Ω-cm. Atthat level of Boron doping the epitaxial layer 12 can resist asubsequent EDP etch used to release the cantilevered beam from wafer 10and thus an etch stop layer is not needed. Preferably, the level ofdoping in layer 12 reduces the resistivity of layer 12 to less than 0.05Ω-cm.

[0041] The structures shown in the drawings has been described in manyinstances with reference to a capital letter ‘E’. However, this shape isnot particularly critical, but it is preferred since it provides goodmechanical support for the cantilevered structure formed primarily bybeam portion of layer 12. Of course, the shape of the supporting andmating structure around cantilever beam 12 can be changed as a matter ofdesign choice and it need not form the perimeter of the capital letter‘E’, but can form any convenient shape, including circular, triangularor other shapes as desired.

[0042] This description includes references to Ti/Pt/Au layers. Thoseskilled in the art will appreciate that this nomenclature refers to asituation where the Ti/Pt/Au layer comprises individual layers of Ti, Ptand Au. The Ti layer promotes adhesion, while the Pt layer acts as abarrier to the diffusion of Si from adjacent layers into the Au. Otheradhesion layers such as Cr and/or other diffusion barrier layers such asa Pd could also be used or could alternatively be used. It is oftendesirable to keep Si from migrating into the Au, if the Au forms acontact, since if Si diffuses into an Au contact it will tend to formSiO₂ on the exposed surface and, since SiO₂ is a dielectric, it hasdeleterious effects on the ability of the Au contact to perform itsintended function. As such, a diffusion barrier layer such as Pt and/orPd is preferably employed between an Au contact and adjacent Simaterial. However, an embodiment is discussed wherein the diffusionbarrier purposefully omitted to form an Au/Si eutectic.

[0043] The nomenclature Au/Si or Au—Si refers a mixture of Au and Si.The Au and Si can be deposited as separate layers with the understandingthat the Si will tend to migrate at elevated temperature into the Au toform an eutectic. However, for ease of manufacturing, the Au/Si eutecticis preferably deposited as a mixture except in those embodiments wherethe migration of Si into Au is specifically relied upon to form Au/Si.

[0044] Many different embodiments of a MEM device have been described.Many more embodiments can certainly be envisioned by those skilled inthe art based the technology disclosed herein. But in all cases the basestructure 4 is united with the cantilevered beam forming structure 2 byapplying pressure and preferably also heat, preferably to cause aneutectic bond to occur between the then exposed layers of the twostructures 2 and 4. The bonding may instead be done non-eutectically,but then higher temperatures must be used. Since it is usually desirableto reduce and/or eliminate high temperature fabrication processes, thebonding between the two structures 2 and 4 is preferably doneeutectically and the eutectic bond preferably occurs between confrontinglayers of Si and Au/Si.

[0045] In operation, the side electrodes are used to create a force onthe cantilevered beam that then oscillates laterally across thesubstrate in response thereto. When the gyroscopic sensor is rotatedabout its axis (i.e. the axis of the cantilevered beam), a Coriolisforce is produced normal to the plane of the substrate. This force isdetected as an oscillating tunneling current by the control electrodesin a servo loop. The servo loop responds by oscillating the controlelectrode voltage for force rebalancing operation at the lateralresonant frequency of the cantilevered beam. The side drive electrodesare preferably fabricated with the cantilevered beam on the firstsubstrate and are bonded to the second substrate at the same time thatthe cantilevered beam is attached. This provides for high alignmentaccuracy between the cantilevered beam and the side electrodes.

[0046] Having described the invention with respect to certain preferredembodiments thereof, modification will now suggest itself to thoseskilled in the art. The invention is not to be limited to the foregoingdescription, except as required by the appended claims.

What is claimed is:
 1. A method of making a MEM tunneling gyroscopecomprising the steps of: (a) defining a cantilevered beam structure,first portions of at least two side drive electrodes and a matingstructure on a first substrate or wafer; (b) forming at least onecontact structure, second portions of said at least two side driveelectrodes and a mating structure on a second substrate or wafer, themating structure on the second substrate or wafer being of acomplementary shape to the mating structure on the first substrate orwafer and the second portions of the side drive electrodes being of acomplementary shape to the first portions of the side drive electrodeson the first substrate or wafer; (c) positioning the mating structure ofthe first substrate or wafer into a confronting relationship with themating structure of the second substrate or wafer; (d) bonding a layerassociated with said mating structure on the first substrate or waferwith a layer associated with the mating structure on the secondsubstrate or wafer; (e) bonding layers associated with said firstportions of at least two side drive electrodes on the first substrate orwafer with layers associated with said second portions of at least twoside drive electrodes on the second substrate or wafer; (f) removing atleast a portion of the first substrate or wafer to release thecantilevered beam structure.
 2. A method of making a MEM tunnelinggyroscope as claimed in claim 1 wherein the second substrate or wafer isformed of silicon.
 3. A method of making a MEM tunneling gyroscope asclaimed in claim 2 wherein the silicon forming the second substrate orwafer is of a single crystalline structure.
 4. A method of making a MEMtunneling gyroscope as claimed in claim 3 wherein the crystallinestructure of the silicon is<100>.
 5. A method of making a MEM tunnelinggyroscope as claimed in claim 4 wherein the silicon is n-type.
 6. Amethod of making a MEM tunneling gyroscope as claimed in claim 1 whereinthe first substrate or wafer is formed of silicon.
 7. A method of makinga MEM tunneling gyroscope as claimed in claim 6 wherein the siliconforming the first substrate or wafer is of a single crystallinestructure.
 8. A method of making a MEM tunneling gyroscope as claimed inclaim 7 wherein the crystalline structure of the silicon in the firstsubstrate or wafer is<100>.
 9. A method of making a MEM tunnelinggyroscope as claimed in claim 8 wherein the silicon of the firstsubstrate or wafer is n-type.
 10. A method of making a MEM tunnelinggyroscope as claimed in claim 1 wherein heat is applied together withpressure between the two substrates so as to cause an eutectic bond tooccur between the two mating structures and between the first and secondportions of the side drive electrodes.
 11. A method of making a MEMtunneling gyroscope as claimed in claim 1 wherein the cantilevered beamstructure is formed by: (a) forming an epitaxial layer of silicon onsaid first substrate or wafer, said epitaxial layer being doped; (b)masking and etching the epitaxial layer of silicon to define a beamstructure disposed on said first substrate or wafer; and (c) removingthe first substrate or wafer by etching.
 12. A method of making a MEMtunneling gyroscope as claimed in claim 11 wherein a contact is formedon an end of said beam structure by depositing a metal through a smallopening in a temporary mask layer, the small opening being sufficientlysmall that the metal being deposited tends to overhang the small openingincreasingly as the deposition of the metal proceeds whereby the contactbeing deposited through the small opening assumes an elongate shape ofdecreasing cross section as the deposition proceeds.
 13. A method ofmaking a MEM tunneling gyroscope as claimed in claim 11 wherein theepitaxial layer is doped with boron at a sufficient concentration toreduce the resistivity of the epitaxial layer to less than 0.05 Ω-cm.14. A method of making a MEM tunneling gyroscope as claimed in claim 13wherein etching accomplished by ethylenediamine pyrocatechol as anetchant.
 15. A method of making a MEM tunneling gyroscope as claimed inclaim 13 wherein a layer of metal, preferably formed of individuallayers of Ti, Pt and Au, is selectively deposited on said epitaxiallayer and sintered at an elevated temperature to form first and secondohmic contacts on said epitaxial layer, said second ohmic contact beingdisposed near a distal end of the beam structure and the first ohmiccontact forming the mating structure on the first substrate or wafer.16. A method of making a MEM tunneling gyroscope as claimed in claim 15wherein a relatively thick layer of metal is deposited and then sinteredon a relatively thin metal layer, a first portion of the relativelythick layer of metal forming the mating structure on the first substrateor wafer and overlying said first ohmic contact and a second portion ofthe relatively thick layer of metal forming a pointed contact at saidsecond ohmic contact.
 17. A method of making a MEM tunneling gyroscopeas claimed in claim 16 wherein the relatively thick layer of metal isTi/Pt/Au.
 18. A method of making a MEM tunneling gyroscope as claimed inclaim 17 further including forming Ti/Pt/Au contacts on said secondsubstrate or wafer, at least one of said contacts on the secondsubstrate or wafer defining the mating structure on the second substrateor wafer.
 19. A method of making a MEM tunneling gyroscope as claimed inclaim 18 wherein the bonding occurs eutectically and the layer forproducing an eutectic bond is provided by a layer of Au—Si eutecticdeposited on the Ti/Pt/Au contact on said second substrate or waferand/or by a layer of Au—Si eutectic deposited on first portion of therelatively thick layer of Ti/Pt/Au on the first substrate or wafer. 20.A method of making a MEM tunneling gyroscope as claimed in claim 1wherein said first substrate includes an etch stop layer and wherein thecantilevered beam structure, the first portions of at least two sidedrive electrodes and the mating structure thereof are disposed adjacentsaid etch stop layer and wherein the cantilevered beam structure isreleased by first etching away the first substrate and then by etchingaway the etch stop layer.
 21. A method of making a MEM tunnelinggyroscope as claimed in claim 20 wherein the second substrate or waferis formed of silicon.
 22. A method of making a MEM tunneling gyroscopeas claimed in claim 21 wherein the silicon forming the second substrateor wafer is of a single crystalline structure.
 23. A method of making aMEM tunneling gyroscope as claimed in claim 22 wherein the crystallinestructure of the silicon is<100>.
 24. A method of making a MEM tunnelinggyroscope as claimed in claim 23 wherein the silicon is n-type.
 25. Amethod of making a MEM tunneling gyroscope as claimed in claim 20wherein the first substrate or wafer is formed of silicon.
 26. A methodof making a MEM tunneling gyroscope as claimed in claim 25 wherein thesilicon forming the first substrate or wafer is of a single crystallinestructure.
 27. A method of making a MEM tunneling gyroscope as claimedin claim 26 wherein the crystalline structure of the silicon in thefirst substrate or wafer is<100>.
 28. A method of making a MEM tunnelinggyroscope as claimed in claim 27 wherein the silicon of the firstsubstrate or wafer is n-type.
 29. A method of making a MEM tunnelinggyroscope as claimed in claim 20 wherein heat is applied together withpressure between the two substrates so as to cause an eutectic bond tooccur between the two mating structures and between the first and secondportions of the side drive electrodes.
 30. An assembly for making a MEMtunneling gyroscope therefrom, the assembly comprising: (a) a beamstructure, first portions of side drive electrodes and a matingstructure defined on a first substrate or wafer; (b) sense electrodes,second portions of the side drive electrode and a mating structuredefined on a second substrate or wafer, the mating structure on thesecond substrate or wafer being of a complementary shape to the matingstructure on the first substrate or wafer; and the second portions ofthe side drive electrodes being of a complementary shape to the firstportions of side drive electrodes on the first substrate or wafer; and(c) a pressure/heat sensitive bonding layer disposed on at least one ofsaid mating structures and on at least one of said first and secondportions of the side drive electrodes for bonding the mating structuredefined on the first substrate or wafer to mating structure on thesecond substrate or wafer and for bonding said first and second portionsof the side drive electrodes together in response to the application ofpressure/heat therebetween.
 31. An assembly as claimed in claim 30wherein the first and second substrates or wafers are formed of silicon.32. An assembly as claimed in claim 31 wherein the silicon forming thefirst and second substrates or wafers is of a single crystallinestructure.
 33. An assembly as claimed in claim 32 wherein thecrystalline structure of the silicon is <100>.
 34. An assembly asclaimed in claim 33 wherein the silicon is n-type.
 35. An assembly asclaimed in claim 30 wherein said first substrate includes a thin etchstop layer and wherein the beam structure, the first portions of sidedrive electrodes and the mating structure thereof are disposed adjacentsaid etch stop layer.
 36. An assembly as claimed in claim 35 wherein thesaid beam structure is undoped and wherein a thin elongate ribbonconductor is disposed on said beam structure.
 37. An assembly as claimedin claim 36 wherein said beam structure is doped to reduce itsresistivity is less than 0.05 Ω-cm.
 38. An assembly as claimed in claim35 wherein the thin etch stop layer is SiO₂.
 39. An assembly as claimedin claim 30 wherein a pointed contact is disposed on an end of said beamstructure.
 40. An assembly as claimed in claim 39 wherein the epitaxiallayer is doped with Boron at a sufficient concentration to reduce theresistivity of the epitaxial layer to less than 0.05 Ω-cm.
 41. Anassembly as claimed in claim 40 further including first and second ohmiccontacts on said epitaxial layer, said second ohmic contact beingdisposed near a distal end of the beam structure and said first ohmiccontact forming the mating structure on the first substrate or wafer.42. An assembly as claimed in claim 41 further wherein said first andsecond ohmic contacts are Ti/Pt/Au contacts.
 43. An assembly as claimedin claim 42 wherein a relatively thick layer of Ti/Pt/Au is disposed onthe first and second ohmic Ti/Pt/Au contacts , a first portion of therelatively thick layer of Ti/Pt/Au being disposed on said first ohmicTi/Pt/Au contact and providing the mating structure on the firstsubstrate and a second portion of the relatively thick layer of Ti/Pt/Auforming a pointed contact on said second ohmic Ti/Pt/Au contact.
 44. Anassembly as claimed in claim 42 further including Ti/Pt/Au contactsdisposed on said second substrate or wafer, at least one of saidcontacts on the second substrate or wafer defining the mating structureon the second substrate or wafer.
 45. An assembly as claimed in claim 44wherein the bonding layer is provided by a layer of Au—Si eutecticdisposed on the Ti/Pt/Au contact on said second substrate and/or by alayer of Au—Si eutectic disposed on the first portion of the relativelythick layer of Ti/Pt/Au on the first substrate or wafer.
 46. An assemblyas claimed in claim 41 further including first and second ohmic contactson said epitaxial layer, said second ohmic contact being disposed near adistal end of the beam structure and said first ohmic contact formingthe mating structure on the first substrate.
 47. A MEM tunnelinggyroscope assembly comprising: (a) a beam structure and a matingstructure defined on a first substrate or wafer; (b) at least onecontact structure and a mating structure defined on a second substrateor wafer, the mating structure on the second substrate or wafer being ofa complementary shape to the mating structure on the first substrate orwafer; and (c) a bonding layer is disposed on at least one of saidmating structures for bonding the mating structure defined on the firstsubstrate or wafer to the mating structure on the second substrate orwafer, the mating structures being joined one to another at said bondinglayer.
 48. A MEM tunneling gyroscope assembly as claimed in claim 47wherein the first and second substrates or wafers are each formed ofsingle crystal silicon.
 49. A MEM tunneling gyroscope assembly asclaimed in claim 48 wherein the crystalline structure of the siliconis<100>.
 50. A MEM tunneling gyroscope assembly as claimed in claim 47wherein the cantilevered beam structure is formed from an epitaxiallayer of silicon on said first substrate or wafer, said epitaxial layerbeing doped with a dopant.
 51. A MEM tunneling gyroscope assembly asclaimed in claim 50 wherein the epitaxial layer is doped with Boron at asufficient concentration to reduce the resistivity of the epitaxiallayer to less than less than 0.05 Ω-cm.
 52. A MEM tunneling gyroscopeassembly as claimed in claim 50 further including first and second ohmiccontacts on said epitaxial layer, said second ohmic contact beingdisposed near a distal end of the beam structure and said first ohmiccontact forming the mating structure on the first substrate or wafer.53. A MEM tunneling gyroscope assembly as claimed in claim 52 wherein arelatively thick layer of metal is disposed on the first and secondohmic contacts, a first portion of the relatively thick layer of metalbeing disposed on said first ohmic contact and providing the matingstructure on the first substrate or wafer and a second portion of therelatively thick layer of metal forming a pointed contact on said secondohmic contact.
 54. A MEM tunneling gyroscope assembly as claimed inclaim 53 further including metal contacts disposed on said secondsubstrate or wafer, at least one of said contacts on the secondsubstrate or wafer defining the mating structure on the second substrateor wafer.
 55. A MEM tunneling gyroscope assembly as claimed in claim 54wherein the bonding layer is provided by a layer of Au—Si eutecticdisposed on the metal contact on said second substrate or wafer and/orby a layer of Au—Si eutectic disposed on the first portion of therelatively thick layer of metal on the first substrate or wafer.
 56. AMEM tunneling gyroscope assembly as claimed in claim 51 furtherincluding first and second ohmic contacts on said epitaxial layer, saidsecond ohmic contact being disposed near a distal end of the beamstructure and said first ohmic contact forming the mating structure onthe first substrate or wafer.
 57. A MEM tunneling gyroscope assembly asclaimed in claim 47 wherein the cantilevered beam structure is formedfrom said first substrate or wafer with a layer of SiO₂ being providedbetween said cantilevered beam structure other portions of said firstsubstrate or wafer.